There is a wide variety of gears in the market. Ultimately it is your own decision and the assessment of your riding style that matters.
Usually, complete gear-sets are installed in Trekkingbikes and Mountainbikes from Univega.
You can find more information in the Tech-specifics.
Categories of Shimano MTB-Gears
Starting with the Top-Model
Deore XT: ambitious Mountainbikers
The Deore XT is the High End Performance gear of Shimano. It captivates through exact and precisely defined switching operation. It is very robust, reliable and durable. Univega, for example, installs them in the Alpina LTD.
The SLX still provides the defined switching operation even in the most demanding terrain. It is a little bit heavier than the Deore XT, but works on the same performance level. Univega, for example, installs them in the E-MTB Renegade S 4.5 Alpine.